1. Field of the Invention
This invention relates to a thermal head, which is a printing component in a printer for thermally carrying out a printing operation onto thermosensible paper or, which utilizes thermal fusion and transfer of ink on an ink donor sheet onto a receiver, or which works as a fixing heater in a copying machine for fixing a powdered ink onto paper.
2. Description of the Related Art
A conventional thermal head Generally employs a localized glaze layer structure (hereinafter referred to as a "partial glaze" structure) for the purpose of effective thermal transmission, which is disclosed, for example, in Japan Patent Publication (SHO)63-35597. Such a conventional thermal head having a partial glaze structure is shown in FIGS. 24-27.
Referring to FIG. 24, a prior art thermal head 20 comprises an alumina substrate 21, a glass glaze strip 22 (i.e. partial glaze 22) printed on the substrate 21 by screen printing, and a heating element strip 23 formed on and along the top of the glaze strip 22. The heating element 23 is connected to driving circuits 25 via conductive patterns 24 formed on the substrate 20 by printing or vapor deposition and made of conductive material such as Gold. The heating element 23 is driven by electrical control of the driving circuits 25 so as to generate heat. The material of the glaze layer 22 is prepared by mixing and melting SiO.sub.2, MgO, CaO, Al.sub.2 O.sub.3, BaO, Sr (strontium), etc., then solidifying and grinding it into powder, and mixing resin and thinner into the powder to make a paste. The paste is printed on the substrate, which is then annealed to form a glaze strip.
FIGS. 25 and 26 show, in the form of a cross-section, Glaze layer strips with different widths, both showing before and after the annealing. In FIG. 25, the width of the paste strip (i.e. paste printing width) is set narrow, while in FIG. 26, the width is relatively large. Generally, when the width "W" exceeds 2.5 mm, the material paste 22' tends to swell at both ends in the width direction, i.e. along the longitudinal sides of the paste strip (see FIG. 26, the swelling portions are indicated by numeral 22A.) For this reason, the printing width of the paste is set to 2.5 mm or less, and preferably, less than 2 mm. The paste 22' printed on the substrate is then annealed at about 1,000.degree. C. The glaze strip is convexly shaped (having a mountain like cross-section) by the inherent viscosity of the material (See the above-mentioned SHO 63-35597).
Generally, as is shown in FIG. 27, the thermal head 20 is manufactured by forming a plurality of thermal heads on an alumina substrate 26 and cutting them out separately.
The smoothly curved cross-sectional shape of the localized glaze layer 22 enables good contact between the thermal head and heated (printed) medium and efficient heat transmission. However, the glaze layer also works as a thermal storage layer, and such a thermal storage phenomenon becomes a disadvantage for high speed printing where cooling and heating operations must be repeated at high speed.
In order to overcome this problem, attempts were made to decrease the heat capacity of the glaze layer strip 22 by making the cross-section area of the glaze strip 22 smaller. However, minute printing of a glaze layer strip having a narrow width on a substrate requires a high level printing technique. Even using the most advanced printing techniques, 0.3 mm is the minimum width.
Although the width of the glaze strip is theoretically defined by the viscosity of the material paste, the actual width is influenced by subtle variations in environmental temperature during the annealing, or the annealing time itself, which results in variations in the resultant glaze layer. Due to this, it was difficult to form a glaze strip having a small and uniform cross sectional area. Especially, maintaining the minimum width of 0.3 mm during mass-production of the thermal head was difficult. Furthermore, although it is preferable to make the height of the partial glaze 22 higher for better contact with the paper, the height (i.e. thickness) of the material paste simultaneously printed on the substrate is also limited by the viscosity of the material paste, and it is very difficult to make the height higher while decreasing the width of the glaze layer strip Thus, this implies limitations in design aspect as well as in printing technique.
In view of the above mentioned limitations, it was proposed to increase the viscosity of the material paste for the purpose of making the width of the glaze layer strip smaller. However, in order to increase the viscosity, impurities added into the material paste are naturally increased. Even if the printing condition of a material paste having high viscosity is good, the impurities, such as binder, are evaporated during the annealing of the printed material paste (in the furnace at 1,000.degree. C.), which results in a uneven surface of the glaze strip 22. The unevenness of the surface of the partial glaze 22 will cause many problems in later processes, for example, a heating element formed on the glaze layer 22 after printing and annealing of a resistance paste is inferior, and the contacting condition between the thermal head and paper is impaired, causing unclear printing. Since such an uneven surface of the glaze layer can be smoothed to some extent by reannealing it at a first annealing temperature, the increase of the impurities may be fairly effective for forming a higher glaze layer strip. However, the amount of the binder added is limited, and it must not exceed the amount of glass component, and it is difficult to make a suitable material paste which can realize a preferable height and width of the glaze layer 22.
The inventor found that as the curvature of the convex surface of the glaze layer becomes large, the contact condition between the thermal head and thermosensible paper becomes better. However, since the curvature of the glaze layer strip is defined by the viscosity and the width of the material paste, it is still difficult to realize a glaze layer strip which satisfies both requirements of larger curvature and smaller heat storage.